1. Field of the Invention
This invention relates generally to video cameras and, more particularly, is directed to an improved arrangement for effecting automatic white balance adjustment in a video camera system of the type having a camera body on which exchangeable lens devices are interchangeably mountable.
2. DESCRIPTION OF THE PRIOR ART
It is known to provide a video camera with a white balance adjusting circuit. Such video cameras generally comprise a lens assembly for projecting a light image on an image pick up element which may be a charge-coupled-device (CCD) and which provides a corresponding video signal supplied to a signal processing circuit for generating color signals. Any white balance errors included in such color signals are intended to be corrected by the white balance adjusting circuit which may be of a closed-loop type or an open-loop type.
In a video camera provided with a white balance adjusting circuit, red, blue and green color signals provided by the signal processing circuit are supplied through respective amplifiers to a matrix circuit which produces therefrom the usual luminance signal and color difference signals. In the case of a white balance adjusting circuit of the closed-loop type, the outputs of the red and blue color signal amplifiers, which have variable gains, are compared in respective operational amplifiers with the output of the green color signal amplifier to provide red and blue compensation signals which are stored in memories and readable from the latter for controlling the gains of the red and blue color signal amplifiers, respectively. In using a video camera having a closed-loop type white balance adjusting circuit, a white object, for example, a white cap covering the lens assembly, is initially placed in the field of view of the camera and, in the event that there is a white balance error, the white balance adjusting circuit provides red and blue compensation signals that are adapted to vary the gains of the red and blue color signal amplifiers in a manner to cancel such white balance error. The red and blue compensation signals are stored in the respective memories of the camera and are thereafter, during normal operation of the camera, supplied from the memories to the respective variable gain amplifiers for maintaining the desired white balance of the color video signal corresponding to a scene that may then be in the field of view of the camera. However, a video camera having a white balance adjusting circuit of the closed-loop type requires that an initial white balance adjustment be performed before each use of the camera for obtaining the red and blue compensation signals to be stored in the respective memories.
It will be seen that, in a white balance adjusting circuit of the closed-loop type, the stored red and blue compensation correction signals are derived from the main image signal path. On the other hand, in a white balance adjusting circuit of the open-loop type, for example, as disclosed in U.S. Pat. No. 4,646,161, and in Japanese laid-open Utility Model Application No. 61-1982, each of which has a common assignee herewith, an external light sensor assembly is provided to convert white light obtained from a white-scattering plate independently of the main image path into red, green and blue detected signals, whereupon, the red and blue detected signals are compared, in respective operational amplifiers, with the green detected signal to continuously provide red and blue compensation signals, respectively, which are employed for controlling the gains of the red and blue color signal amplifiers for correcting white balance errors. Thus, the white balance is continuously adjusted for obtaining the optimum white balance regardless of fluctuations in the photographing conditions, such as, changes in the ambient illumination and the like. Further, in the case of a white balance adjusting circuit of the open-loop type, the camera operator is freed from the annoying and complicated procedure of providing an initial white balance adjustment for obtaining the compensation or correcting signals to be stored in the memories before the actual use of the camera for providing a picture.
However, in the case of a video camera having a white balance adjusting circuit of the open-loop type, since the white light used for obtaining the compensating or correcting signals is received by an external light sensor arrangement, that is, from outside the main image signal path, as described above, white balance errors introduced in the imaging video signal, for example, by exchanging one lens assembly for another lens assembly having different spectral characteristics, cannot be corrected.